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Introduction

Figure 1 Regional total magnetic intensity and Bouguer gravity images showing the location of the Coompana Province with respect to the surrounding geological provinces. Also highlighting the large reversely polarised Coompana Magnetic Anomaly.

The Coompana Province is a completely buried crustal block that occurs at the nexus of the West, South and North Australian cratons. However, very little is known about the region. The Geological Survey of Western Australia drilled a series of holes into the westernmost part of the province, which provided some insights into the architecture and geological history (Spaggiari and Smithies 2015). Yet this left a large part of the province as unknown. This was addressed with a recent program to acquire and interpret geological, geochemical and geophysical data for the Coompana Province in western South Australia (Fig. 1). Results and some interpretations have been released in a recent MESA Journal article and a workshop abstract volume (Dutch, Pawley et al. 2018; Dutch, Wise et al. 2018). This paper will expand on these publications and present the broader implications of the work that was undertaken in the Coompana Province. Whilst insights into the geology of the eastern Coompana Province have been hinted at with existing data from sparse drilling (Neumann and Korsch 2014; Fraser and Neumann 2016; Travers 2015) and correlations with the western Coompana, Madura and Musgrave provinces, the eastern Coompana Province was still a gap in the understanding of Proterozoic Australia (Fig. 2; Spaggiari et al. 2015).

Data acquisition in the South Australian part of the Coompana Province commenced in 2013 with the 13GA-EG1 seismic line, linking the central Gawler Craton, with the Albany–Fraser Orogen in Western Australia (Dutch, Pawley and Wise 2015). The seismic line imaged, for the first time, the architecture of the western margin of the Gawler Craton and the contact with the largely unknown basement rocks of the Coompana Province (Doublier et al. 2015). Complementing the deep-crustal imaging seismic line was a co-located broadband magnetotellurics (MT) profile (Thiel, Wise and Duan 2015; Thiel, Wise and Duan 2018). In the Coompana Province the seismic and MT profiles image a complex story of major crustal-scale structures and extensional fabrics exploited and overprinted by multiple generations of intrusive magmatism (Pawley, Wise, Dutch et al. 2018; Thiel, Wise and Duan 2018; Wise and Thiel 2018).

Regional potential field surveys were acquired in 2015 (aeromagnetics and radiometrics; Heath, Reed and Katona 2015) and 2017 (ground gravity; Heath and Wise 2017). Preliminary interpretation of the aeromagnetic imagery showed a number of broad geophysical domains and possible reversely magnetised intrusions (Wise, Pawley and Dutch 2015) that would be the focus of the 2017 Coompana Drilling Project (Fig. 3; Dutch, Pawley et al. 2018).

In this paper we will synthesise the results of the large body of precompetitive data acquired (Dutch, Pawley et al. 2018; Dutch, Wise et al. 2018) and propose a geological framework of the basement to the eastern Coompana Province. We will also present a model for the formation of the prominent and enigmatic Coompana Magnetic Anomaly, and discuss the metallogenic potential of the region.

Geological framework

The Coompana Drilling Project (Fig. 3; Dutch, Pawley et al. 2018) provided fundamental geological controls that could be used to constrain the geophysical interpretations (e.g. seismic: Pawley, Wise, Dutch et al. 2018; and aeromagnetics: Wise, Pawley and Dutch 2015). The lithological and petrographical information in the drill core (Pawley, Wise, Jagodzinski et al. 2018) was combined with geochronology (Jagodzinski and Bodorkos 2018) to create a stratigraphy (Wise, Pawley and Dutch 2018a). Geochemical and isotopic analysis of the core then allowed the geological history of the region to be unravelled (Dutch 2018; Hartnady et al. 2018). Potential field datasets were then used to determine the spatial extents of the units, resulting in an interpretive solid geology map (Fig. 4; Wise, Pawley and Dutch 2018b). Fundamentally, the evolution of the eastern Coompana Province can be divided into four main geodynamic events (Fig. 5), which are summarised in this section. For more detail, the reader is referred to Dutch, Wise et al. (2018).

1 Oceanic crust development

Nd and Hf isotope data from the Coompana Province (Dutch 2018; Hartnady et al. 2018; Kirkland et al. 2017) have revealed a putative c. 2000–1900 Ma mantle extraction event that appears to be consistently represented in rocks from across the province and the neighbouring Musgrave and Madura provinces. As signatures of this event appear relatively time-constrained and of a juvenile nature, Hartnady et al. (2018) and Kirkland et al. (2017) interpret this event to represent oceanic crust development outboard of the Gawler Craton in the period c. 2000–1900 Ma, with the crust subsequently reworked and destroyed during later magmatism.

2 Prolonged arc–subduction cycles

The oldest rocks dated in the Coompana Province are the c. 1618 Ma Koomalboogurra Suite, of the Toolgana Supersuite (Dutch 2018; Jagodzinski and Bodorkos 2018; Wingate et al. 2015a). The monzogranitic orthogneisses are comparable in age and geochemistry to the St Peter Suite in the southern Gawler Craton, and have been interpreted to represent subduction-related granites, developed on the edge, or outboard of the Gawler Craton (Dutch 2018; Swain et al. 2008). The c. 1526 Ma migmatitic orthogneisses of the Bunburra Suite represent a newly reported magmatic event in this region, with juvenile isotopic character and primitive geochemical signatures (Dutch 2018; Jagodzinski and Bodorkos 2018). Dutch (2018) interprets the Bunburra Suite to be derived from a subduction-enriched lithospheric mantle source.

As age constraints on the subduction-related rocks (above) appear to young from east to west – c. 1618 Ma in the east, to c. 1400 Ma and c. 1390 Ma in the west (Madura Province: Smithies et al. 2015; Wingate et al. 2015b; Musgrave Province: Smithies et al. 2010) – we interpret the Coompana and Madura provinces to have their origin in broadly westward-migrating (i.e. back-stepping), approximately north–south-trending arc ribbons developed on the proposed oceanic crust between the Gawler Craton and Yilgarn Craton (Dutch et al. 2016).

3 Intracontinental meltdown

In the period c. 1200–1070 Ma, magmatic rocks with a wide variety of lithology, age and compositions were intersected in drillholes of the Coompana Drilling Project (Dutch, Wise et al. 2018).

The relatively juvenile ɛHf1174–1140 Ma values of the Merdayerrah Shoshonite and Koonalda Suite melts suggest little or no assimilation of any pre-1900 Ma crust, and appear to be the product of mantle input and assimilation of crust similar to the Bunburra and Koomalboogurra suites (Dutch 2018).

In contrast, mantle melts at c. 1074 Ma (Giants Head Suite: Jagodzinski and Bodorkos 2018) show evolved (strongly negative) ɛHf1074 Ma values, indicating that contamination by a >1900 Ma crustal substrate is required (Dutch 2018). We therefore interpret a highly reflective lower crustal unit in the seismic section, coincident with the top of a subvertical mantle conductor (Fig. 6) represents a relic of possible Gawler Craton crust beneath the Coompana Province, and is the contaminant in ascending mantle melts.

The varied geochemical signatures exhibited by mafic rocks of the c. 1074 Ma Giants Head Suite (Dutch 2018) are suggestive of a heterogeneous lithospheric mantle. Evidenced by the significant negative gravity anomaly associated with the Coompana Province (Fig. 1), it is possible that delamination of the mafic residual components of c. 1200–1140 Ma MASH chambers in the lower crust into the lithosphere provided a catalyst for melting of heterogeneous source material for the c. 1074 Ma magmatism.

Development of thick continental lithosphere and metasomatism of the lithospheric mantle associated with c. 1620–<1500 Ma arc development, and a widespread mantle heat source at c. 1200 Ma were the likely catalysts for a >100 My period of magmatism between c. 1200 Ma and c. 1074 Ma.

Coompana Magnetic Anomaly

The source of the Coompana Magnetic Anomaly, a ~50 km wide, circular, reversely polarised anomaly in the southern Coompana Province has been the topic of debate since it was first identified by widely spaced aeromagnetic data in the early 1970s. Drilling in the early 1980s targeted ~1–2 km wide satellite anomalies, with mafic volcanics and intrusive rocks being returned from below the basement unconformity (Shell Co. of Australia Ltd 1983; Carpentaria Exploration Co. Pty Ltd 1982a, 1982b). An early attempt to ascertain the age of these volcanics using Sm–Nd isochrons gave a poorly constrained age of c. 859 ± 66 Ma (Travers 2015). Accurate U–Pb dating of these mafic rocks was not possible until drillhole CDP002 (Dutch, Jagodzinski et al. 2017) returned massive olivine dolerite and microgabbro from beneath the basement unconformity. A fractionated interval close to the top of the basement interval returned a small population of zircon, and was dated with a magmatic crystallisation age at 1074 ± 6 Ma (Jagodzinski and Bodorkos 2018). The age of these rocks indicates that they are part of the c. 1078–1070 Ma Warakurna Supersuite (Howard et al. 2011; Wingate, Pirajno and Morris 2004), significantly increasing the extent of the Warakurna Large Igneous Province (Alghamdi et al. 2018; Wingate et al. 2004). Magnetisation studies on the reversely polarised anomalies caused by the satellite bodies intersected in previous drilling and in CDP002 suggest that magnetisation directions, whilst variable, are consistent with an extended period of magmatism, rather than multiple discrete events (Foss et al. 2018). This therefore implies that satellite bodies, and the Coompana Magnetic Anomaly, are all likely to be temporally related to the CDP002 olivine dolerite–microgabbro.

Of particular interest are the gravity and magnetic signatures of the satellite bodies and the Coompana Magnetic Anomaly. The satellite bodies have reversely polarised magnetic anomalies that are spatially associated with positive gravity anomalies, implying that the causative body is both strongly magnetic and dense, e.g. the olivine dolerite–microgabbro in CDP002. In contrast, the Coompana Magnetic Anomaly displays a similarly reversely polarised high intensity magnetic anomaly, but is not spatially associated with a positive gravity anomaly (Foss et al. 2018). As the body causing the Coompana Magnetic Anomaly is strongly magnetic, we propose that initially, this was a mafic–ultramafic body that has undergone a process to remove the high-density component.

Foundering vs serpentinisation, removal vs alteration

Figure 7 Schematic cartoons of the proposed alternative models generating the unusual signatures of the plutonic body interpreted to be the cause of the Coompana Magnetic Anomaly. (a) Foundering model, where progressive removal of a dense cumulate reduces the net density of the pluton. (b) Serpentinisation model, where fluid infiltration at the intersection of fault–shear systems alters the dense basal layer of the differentiated pluton, producing serpentinite + magnetite.

Whilst an increase in the thickness of a sedimentary succession is required to satisfy low-density responses at the cover–basement interface (Foss et al. 2018), a process to significantly reduce the density of a mafic–ultramafic magmatic body is required to achieve the current density-neutral state with the surrounding host rocks. In both cases, pluton differentiation and stratification is required to have concentrated a mafic–ultramafic cumulate towards the base of the magma chamber, and an upper layer of predominantly plagioclase and magnetite.

We propose two possible, not mutually exclusive, mechanisms for the apparent lack of this layer remaining in situ – foundering and serpentinisation (Fig. 7).

Foundering. As the denser cumulate phase built up, gravitational instabilities developed between the cumulate layer(s) and the less-dense granitic–gneissic host rocks within the weakened thermal aureole of the intrusion (Roman and Jaupart 2016). Negative buoyancy and associated Rayleigh-Taylor instabilities enabled the dense cumulate phase to progressively founder through feeder zones and weakened host rocks, thereby physically removing the dense material from the crystallising pluton (e.g. Glazner 1994; Roman and Jaupart 2016). As no significant long-wavelength (lower crustal) positive gravity anomaly is observed in the Coompana Province (Fig. 1), it is possible that complete removal of the foundered cold, dense cumulate into the mantle was achieved.

Serpentinisation. Pervasive alteration of the mafic–ultramafic cumulate could, if on a large enough scale, reduce the density of the basal layer to the plutonic body, whilst also generating magnetite, required for the high-intensity magnetic anomaly. Such alteration would require significant volumes of fluid to produce serpentinite on this scale. The Coompana body sits at the intersection of the crustal-scale Palinar Shear Zone and a significant NW–NNW-trending structure set. Fluids may have been focused at the intersection of these structures, enabling serpentinisation. Significant pre- and post-magmatic hydrothermal alteration is observed within the Palinar Shear Zone to the southwest (CDP006; Pawley, Wise, Jagodzinski et al. 2018).

Metallogenic implications

The program of data acquisition in the Coompana Province has implications for the prospectivity of the region, as prior to 2013, the sum total of prospectivity indicators were some interesting, but poorly resolved geophysical anomalies in a region entirely blanketed by sedimentary cover. Systematic data acquisition and development of the geological framework described above has identified several factors that raise the prospectivity of the region:

The cover thickness has been constrained by drillholes and multiple geophysical techniques (Foss et al. 2017, 2018; Meixner et al. 2018). Cover thickness is less than 400 m for a large region in the southern Coompana Province, and decreases to ~200 m in places, highlighting the accessibility of the basement rocks.

Geochronology has revealed that mafic rocks of the Giants Head Suite (Dutch 2018; Jagodzinski and Bodorkos 2018; Wise, Pawley and Dutch 2018a) are temporal equivalents of the Giles Complex in the Musgrave Province, and part of the larger Warakurna Supersuite. The Giles Complex is host to the Nebo-Babel deposit in the western Musgrave Province (Godel et al. 2011). The temporal (and genetic) association with the Giles Complex raises the Ni–Cu – platinum group elements potential of the Giants Head Suite.

Conclusion

The Coompana Project represents a major precompetitive geoscience program in a covered, greenfield region which has seen very little scientific or exploration attention. The results of this significant precompetitive geoscience data acquisition program and workflow have been synthesised into a new geological and geodynamic framework that puts the Coompana Province into regional context and fills a knowledge gap in our understanding of Proterozoic Australia. From the data, we are able to recognise four main geodynamic events affecting the Coompana Province, and relate them to the surrounding Proterozoic terranes in the Musgrave and Madura provinces. The development of a geodynamic framework and broader links with surrounding metallogenic provinces have allowed us to identify key indicators of the mineral prospectivity for this previously unknown region.

Fraser GL and Neumann NL 2016. Under the Nullarbor: new SHRIMP U-Pb zircon ages from the Coompana, Madura and Albany-Fraser provinces, and Officer Basin, western South Australia and eastern Western Australia: July 2014 - June 2015, Record 2016/16. Geoscience Australia, Canberra.